The present invention relates to an image forming apparatus such as a printer, a copying machine, a facsimileing machine, and the like which is provided with a cleaning apparatus for cleaning a surface of an image bearing member.
In the field of a cleaning apparatus for an image forming apparatus such as a printer, a copying machine, a facsimile machine, and the like, a cleaning apparatus having a cleaning blade as a cleaning member for cleaning an image bearing member has been known.
In an electrophotographic image forming apparatus, for example, a toner image is formed on a photoconductive drum (image bearing member) through a plurality of image forming processes: a charging process, an exposing process, and a developing process. The toner image is transferred onto a transfer medium (for example, recording paper, OHP sheet, or the like) from the photoconductive drum through a transferring process. During this transferring process, all the toner, of which the toner image on the photoconductive drum is formed, is not transferred; a small amount of the toner remains on the peripheral surface of the photoconductive drum. The toner remaining on the peripheral surface of the photoconductive drum (which hereinafter will be referred to as residual toner) is removed by the above described cleaning blade.
Referring to FIG. 9, the edge 61a of a cleaning blade 61 is placed in contact with the peripheral surface of a photoconductive drum 11, so that the residual toner adhering to the peripheral surface of the photoconductive drum 11 is scraped away by the cleaning blade 61.
However, the conventional cleaning apparatus having the cleaning blade 61 involves the following problems.
Also referring to FIG. 9, as the residual toner is scraped away by the edge 61a of the cleaning blade 61 placed In contact with the peripheral surface of the photoconductive drum 11, it agglomerates in the adjacencies of the edge 61a. Normally, as the agglomeration of the residual toner grows to a certain size, it falls off into the cleaning apparatus shell (unshown) of the cleaning apparatus, so as not to create a problem.
In recent years, however, it became evident that, due to the increase in the peripheral velocity (process speed) of the photoconductive drum 11 resulting from the increase in the speed of an image forming apparatus, the agglomeration of the toner kept on growing in size without falling, and that some of the toner particles slipped through the nip N (cleaning portion) between the edge 61a of the cleaning blade 61 and the photoconductive drum 11, and transferred onto a transfer medium (in the form of a sheet), forming stripes across the medium, during the following image formation cycle. The residual toner having slipped through the nip appears in the form of strips on the recording material in the next image forming operation.
As for a method for improving the cleaning performance of a cleaning blade, a few have been disclosed. For example, Japanese Laid-open Patent Applications 6-4014 and 11-174922 disclose a method which causes a cleaning blade to vibrate with the use of a piezoelectric element. This method, however, suffers from the following faults. That is, a piezoelectric element is attached to a cleaning blade, which deteriorates as its cumulative usage increases, and therefore, must be replaced. As the deteriorated cleaning blade is replaced with a fresh one, the piezoelectric element is replaced together with the deteriorated cleaning blade, resulting in cost increase, since the piezoelectric element is attached to the deteriorated cleaning blade. Further, it is difficult to vibrate sufficiently to remove the grown agglomeration of the residual toner. The Japanese Laid-open Patent Application 9-160455 discloses a method which uses impact to vibrate a cleaning blade. This method, however, suffers from the following problem: a certain behavior of a cleaning blade which occurs as the cleaning blade is vibrated by impact may allow the residual toner to slip through the nip N, although it may be possible to vibrate vigorously enough to remove the grown agglomeration of the residual toner.
However, with the prior art described above it is necessary to impart sufficient vibration to the cleaning blade in order to sufficiently remove the residual toner adjacent the edge of the cleaning blade, and the following problems arise.
Referring to FIG. 9, when a strong vibration is imparted to the cleaning blade 61, the residual toner coagulated at the edge 61a moves, due to the vibration, to a station 61b beyond the nip N, that is onto the cleaning blade 61 downstream of the nip N with respect to the moving direction of the surface of the photosensitive drum 11.
In the first image forming process after the vibration imparting step, the toner at the position 61b beyond the nip N of the cleaning blade 61, jumps at the electrostatic latent image on the photosensitive drum 11 after passing through the edge 61a of the cleaning blade 61, that is, the remaining latent image on the photosensitive drum 11 after the transfer of the toner image. With the result of an image defect in the image after one rotation of the photosensitive drum 11.
The inventors investigations have revealed the mechanism of the occurrence of the image defect attributable to the vibration imparted to the cleaning blade 61.
The toner deposited on the downstream portion 61b of the edge of the cleaning blade 61, keeps retaining the electric charge given by the developing means during the developing operation. Therefore, depending on the intensity of the electric field formed by the potential of the photosensitive drum 11 passing by the cleaning blade 61, the toner deposited on the downstream portion 61 of the edge is capable of jumping at the photosensitive drum 11.
An electric field formed between the developer carrying member and the photosensitive drum 11 which are electroconductors close to each other, as exists in the developing zone of the developing means, is very strong. However, there is no electrode closely opposed to the photosensitive drum 11 at the downstream portion 61b of the edge of cleaning blade 61, and therefore, even if the surface potential of the photosensitive drum 11 is the image dark portion potential, the charge which is strong and causes the toner to jump does not exist.
However, in the case of an electrostatic latent image, such as a linear image, for example, which has a bottom of potential at which the potential abruptly changes, a closed electric field is produced locally at the bottom of the potential with the result that toner deposited on the downstream portion 61b of the edge of the cleaning blade 61 is caused to jump at the photosensitive drum 11 by the closed electric field.
In this manner, the toner deposited on the downstream portion 61b of the edge of the cleaning blade 61 due to the vibration imparted to the cleaning blade 61, jumps at the portion having the intense closed electric field such as the edge portion of a line image which may be included in the electrostatic latent image formed on the photosensitive drum 11 in the next image forming operation. It is transferred onto the transfer material together with the image after one rotation of the photosensitive drum 11. This produces a defect in the image.
Accordingly, it is a principal object of the present invention to provide an image forming apparatus in which a cleaning power of the cleaning member is maintained stably, and the possible problem arising from the vibration imparted to the cleaning member.
According to the present invention, there is provided an image forming apparatus comprising an image bearing member; driving means for driving said image bearing member; latent image forming means for forming an electrostatic latent image on said image bearing member; developing means for developing the electrostatic latent image on said image bearing member with a developer; a cleaning member, contacted to said image bearing member, for cleaning a surface of said image bearing member; vibration imparting means for vibrating said cleaning member; control means for causing said latent image forming means to form an electrostatic latent image pattern for electrostatically attracting the developer deposited on said cleaning member and for causing said driving means to move the electrostatic latent image pattern to a contact portion where said cleaning member is contacted to said image bearing member.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.